Since chapter 3 is possibly right around the corner considering its already been 8 months, I came up with some predictions for what could be in the next chapter:
Blue doesn't show up until the end, since the ending cutscene of the second chapter had him sleeping, and the scientist worrying they'll be late, blue not might appear until the last few hours.
Pink, scrapped from the first, and second chapter, surely he'll make an appearence in the third, right?
Friendly Rainbow Friend
Old man reveals identity
Scientist reveals identity
Purple is some ocean animal, we've seen him always near a bunch of water, my best guess is that hes a squid, even though he does have humanoid arms, those are probably the two tentacles.
Lab setting, not much proof on this one.
Old man returns, although he did leave us on that cart ride, i suspect he'll return by walkie-talkie
Lime, in chapter 2 we got 2 new rainbow friends, Yellow, a close color to Orange, and Cyan, a close color to Blue, in chapter 3, i think that we should have a close color to Green, which is Lime.
The REAL Red, the scientist isn't actually Red, My personal opinion is he's trying to take the identity of Red so that the rainbow friends don't attack him.
Ultimate Custom Night??? It would be so cool to make your own "night" considering the large amount of rainbow friends there already are.
Old man betrays us, if you watched the latest rainbow friends game theory video, you'll see what I mean.